Various abbreviations that appear in the specification and/or in the drawing figures are expanded as follows:    3GPP third generation partnership project    ACK acknowledge    aGW access gateway    ARQ automatic repeat request    DL downlink    EUTRAN evolved UTRAN    eNB EUTRAN Node B (evolved Node B)    FDD frequency division duplex    FDS A frequency division multiple access    FS frame structure    HARQ hybrid automatic repeat request    HSPA high speed packet access    LTE long term evolution    MAC medium access control (layer 2, L2)    MCS modulation and coding scheme    NACK negative acknowledge    NDI new data indicator    Node B base station    OFDMA orthogonal frequency division multiple access    PDCCH physical downlink control channel    PDU protocol data unit    PHICH physical hybrid-ARQ indicator channel    RTT round trip time    RU resource unit    SAW stop and wait    SC-FDMA single carrier, frequency division multiple access    TDD time division duplex    TTI transmission time interval    UE user equipment    UL uplink    UTRAN universal terrestrial radio access network    VoIP voice over internet protocol
A proposed communication system known as evolved UTRAN (E-UTRAN, also referred to as UTRAN-LTE or as E-UTRA) is currently under development within the 3GPP. The agreement at the time of this invention is that the DL access technique is OFDMA, and the UL access technique is SC-FDMA.
One specification of interest to these and other issues related to the invention is 3GPP TS 36.300, V8.1.0 (2007 June), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Access Network (E-UTRAN); Overall description; Stage 2 (Release 8).
There has been discussed several HARQ issues related to LTE.
For example, for LTE a hybrid ARQ (HARQ) protocol that is proposed for use is similar to that of HSPA, namely N parallel hybrid ARQ processes, each implementing a SAW protocol. Each hybrid ARQ process has a certain amount of soft buffer memory in order to implement soft combining.
Further, the number of hybrid ARQ processes should be selected to be as small as possible to minimize the delays associated with HARQ re-transmissions. For FDD the number of processes mainly depends on processing delays. However, for the case of TDD the number of processes required also depends on how sub-frames are allocated to the UL and DL. Considering the processing delays and DL/UL sub-frame configuration, one example of UL HARQ mapping in TDD is shown in FIG. 1. In this context “DSUUU” means 1 DL sub-frames, 1 special sub-frame and 3 UL frames in 5 ms, and the other cases are similar.
It can be further noted that synchronous HARQ has been selected for the LTE UL, so the HARQ process identifications (IDs) in the timeslots numbered as 1, 2, 3, or 4 are shown in darker greyin FIG. 1 are rigorously in order (sequential), and re-transmission must take place within the same HARQ process as its initial/new/first transmission.
Semi-persistent scheduling has also been agreed to for use in LTE, in particular for VoIP service, wherein initial/new transmissions of voice packets are persistently allocated (a set of resources in every 20 ms are predefined) and re-transmissions of packets are dynamically scheduled by Layer 1/Layer 2 signaling.
In semi-persistent scheduling, an initial transmission of voice packets is assumed to always have a higher priority than a re-transmission. Reference in this regard can be made to R2-070476, 3GPP TSG-RAN WG2 Meeting #57, 12-16 Feb. 2007, St. Louis, Miss., USA, “Uplink Scheduling for VoIP”, Nokia.
In some cases of TDD (especially for the cases “DSUUD”, “DSUDD”, “DSUUUDDDDD” and DSUUDDDDDD”)), and due to the characteristic of HARQ process mapping, for one user the re-transmission of some packets can collide with initial transmissions of later packets, that is to say, the re-transmission of some packets and initial transmissions of other packets for one user are all located into the same HARQ process.
For example, and considering the “DSUDD” case in FIG. 1 where there is only one UL sub-frame in the 5 ms interval, and there are a total of two HARQ processes, all of the (re)transmissions of one users' voice packets are in process #1 if the initial/new transmissions are allocated in process #1 (see FIG. 2 in this regard). In FIG. 2, PXY implies the Yth retransmission of Packet X. Due to the number of (re)transmissions ongoing in process #1, in some timeslots it is difficult to determine which packet(new transmission or retransmission) is to be transmitted (indicated as ‘?’ in FIG. 2). Further still, and even if process #2 of this user is empty and free, this user cannot transfer its high traffic load (some number of packets) to process #2 due to restrictions imposed by synchronous HARQ and semi-persistent scheduling. As a result the resource efficiency of this particular user is low. Furthermore, some packet delay is inevitable.